1. Field of the Invention
The present invention relates to a nonaqueous electrolyte battery. The invention is applicable to both a nonaqueous electrolyte primary battery and a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Electronic appliances are becoming smaller in size and lighter in weight recently owing to rapid technical progress in the field of electronics. As a result, cordless and portable electronic appliances are being more prevalent, and secondary batteries used as a driving source are demanded to be smaller in size, lighter in weight, and higher in energy density. To meet such demands, lithium secondary batteries of high energy density are being developed. Lately, ultrathin lightweight secondary batteries using a case of laminate film containing aluminum are being developed and sold on market.
The secondary battery using a case of a laminate film containing aluminum changes in battery thickness due to volume expansion and contraction of an electrode by charging and discharging. As a result, the battery may be deformed, the electrode interval widened, which increases the battery resistance, thus the battery characteristics may be lowered.
To avoid such problems, negative electrode active materials are proposed in Jpn. Pat. Appln. KOKAI Publication No. 9-199179. In the publication, the negative electrode active material is made of lithium titanate or a carbon material of which spacing of (002) plane by an X-ray wide angle diffraction method is 3.7 Å or more, whereby it is intended to decrease expansion and contraction of the electrode due to reversible intercalation and deintercalation of lithium, thereby reducing expansion of an electrode group. Lithium titanate is hardly changed in volume due to charging and discharging, and is hence very small in change of battery thickness. By using this compound in the negative electrode active material, the above problems can be solved.
However, since lithium titanate is poor in conductivity, when the negative electrode is manufactured without adding an electronic conductor as in the publication, the battery is inferior under heavy currents. When the conductivity of the negative electrode active material is low, by adding an electronic conductor made of carbon material to the negative electrode active material, it is known that the high rate characteristic is improved. However, in a nonaqueous electrolyte battery having a negative electrode containing an electronic conductor made of a carbon material and lithium titanate, when the battery is stored in a high temperature environment, or when charging and discharging operation is repeated in a high temperature environment, the electronic conductor reacts with an electrolysis solution, and a large volume of gas is generated, thus the battery characteristics are extremely lowered.